It seems to me that tour bikes of any vintage are pretty much the same except for a few details, while MTB's vary widely over the years. An old school rigid frame MTB really seems like a tour bike on steroids. They had all the attachment points, plenty of fender clearance, 41"-42" wheelbase, quality built strong frames, long chainstays, etc.
My 520 has a wheelbase of 41" while my 1988 Trek 830 had (deceased) 42+. My current roadified MTB, a '92 Trek 930, has a 42" spread. The 520 has nearly 18" in the chainstays, but the MTB's have 17-3/4".
I'm not sure speed is an issue on a tour, especially when the dif will be minimal.
One thing I feel should be mentioned is the apparent higher torque I seem to get with the longer cranks on the MTB, BioPace notwhithstanding.
Handlebars are a personal choice, like saddles, so I won't speak to those points. The percieved advantage of 26" wheels is also kind of a given, especially if you have to replace a tire in some place that doesn't have 700's. Typically, 26" tires are much easier to lever off and on, too. Wrestling with a tight 700 in the dark and rain can be a test.